Before commencing excavation or other work where electrical cables, fiber optic cables or other utilities ducts or pipes are buried, it is important to determine the location of such buried cables or pipes to ensure that they are not damaged during the work. It is also useful to be able to track a path of buried cables or pipes. Current carrying conductors emit electromagnetic radiation which can be detected by an electromagnetic antenna. If fiber optic cables or non-metallic utilities ducts or pipes are fitted with a small electrical tracer line, an alternating electrical current can be coupled into the tracer line which in turn radiates electromagnetic radiation. It is known to use detectors to detect the electromagnetic field emitted by conductors carrying alternating current.
One type of such detector works in one of three modes. These modes are classified as either passive or active modes, the passive modes being ‘power’ mode and ‘radio’ mode which use signals that are already present—mains power signals and submarine VLF (very low frequency) communications. Each mode has its own frequency band of detection.
Aspects of the disclosed subject matter relate to the active mode.
In the active mode, a signal transmitter couples an alternating magnetic field of known frequency and modulation, in a buried conductor. The signal transmitter may be directly connected to the conductor. Where direct connection access is not possible, a signal transmitter may be placed near to the buried conductor and an alternating current signal may be induced in the conductor by an alternating magnetic field produced by the signal generator. The buried conductor radiates an alternating magnetic field corresponding to the signal produced by the signal transmitter.
The choice of signal frequency is an important factor for effective tracing and identification of buried lines, and there is no single frequency that covers all conditions. For single instruments to be used by relatively non-technical personnel there is no option but to make a compromise, and choose a single frequency high enough to give good performance in the induction mode, but not so high that it will not travel far enough. Active signals between 8 kHz and 33 kHz are commonly used for these applications.
33 kHz is considered to be a good general purpose signal frequency suitable for finding many buried cables and metallic pipes. For short lengths of cable, for example telecom spurs crossing a subscriber's premises, a signal frequency of 33 kHz does not provide sufficient signal to give a good quality locate. This is because the signal return path impedance is high, being predominantly capacitive; the shorter the cable, the lower the capacitance to earth and hence the higher the impedance at a particular frequency. The high impedance results in a small current in the cable.
In this situation a better locate signal quality can be obtained using a higher signal frequency. Multi-frequency locators and transmitters are available having suitable high frequency operating modes, e.g. 66 kHz, 83 kHz and 131 kHz. These products require the operator to select a suitable signal frequency, necessitating a higher degree of operator training and greater expertise than possessed by typical users. Dedicated single-frequency locators exist that are optimized for finding telecoms cables, but these are less well suited to general cable and pipe locating as high frequency signals dissipate rapidly with distance along a typical cable or pipe.